1. Field of the Invention
The present invention relates to a radio communication system and a method thereof which realize a space division access by dynamically generating the antenna beam at a base station.
2. Description of the Prior Art
FIG. 13 schematically shows the structure of a conventional base station disclosed in Japanese Laid-open patent publication No. 7-170561. A base station 1301 as shown in FIG. 13 comprises a DBF antenna 1302, a baseband unit 1303, a radio unit 1304 and a tracking control unit 1305.
The operation of the base station will be briefly explained below. The tracking control unit 1305 has a function of tracking each moving mobile station and instructing the DBF antenna 1302 to change the direction of the beam. The DBF antenna 1302 functions to generate a plurality of arbitrary beams in accordance with the instructions sent from the tracking control unit 1305.
With these functions, the base station is capable of realizing a space division access by using the same frequency for each mobile station and providing a high-speed transmission and effective utilization of frequencies.
In a conventional radio communication system, when mobile stations move and come close to other stations, the beams emitted by each mobile station may overlap in the air. This raises a problem that the space division access cannot be realized. To avoid this problem, the conventional radio communication system adopts a frequency division access, namely, the system deals with this problem by changing a frequency of each beam. The operation of the conventional radio communication system will be explained below with reference to FIGS. 14A and 14B.
In FIGS. 14A and 14B, a reference numeral 1401 shows a base station with a DBF (Digital Beam Forming) antenna (which is also called an adaptive antenna), 1402, a mobile station A, 1403, a mobile station B, 1404, the beam to the mobile station A, 1405, the beam to the mobile station B, 1406, the beam to the mobile station A used in a frequency division access and 1407, the beam to the mobile station B used in a frequency division access.
As shown in FIG. 14A, when the mobile stations A and B are separately located with each other, these mobile stations can access the base station by a space-division access method which uses the beams. In FIG. 14A, the beams 1404 and 1405 have the same frequency (f1). If it is detected as shown in FIG. 14B that one mobile station moves and comes close to the another mobile station (both of which use the same frequency), then a frequency used for the beam to the mobile station A (1406) is changed to the one that is different from a frequency used for the beam to the mobile station B (1407), resulting in separation of signals received from each mobile station.
However, there is a problem that the larger the number of frequencies used in one base station, the lower the efficiency of frequency utilization.
The present invention has been made to solve the above-mentioned problem. It is an object of the present invention to provide a communication system and a method thereof for using frequencies effectively even if the mobile stations move and become closely located with one another.
The present invention provides a radio communication system which is provided with a plurality of mobile stations and a base station connected to the mobile stations via a radio, said plurality of mobile stations are served by the same base station and are using the same frequency, the system comprising detection means for detecting whether said plurality of mobile stations are closely located with one another and said base station is impossible to provide a space-division access system using an antenna-beam forming function and change-over means for causing said plurality of mobile stations and said base station to change their access systems from the space-division access system to a time-division access system if it is detected by said detection means that said base station is unable to provide the space-division access system, wherein said plurality of mobile stations and said base station successively use said same frequency for implementation of the time-division access system.
The radio communication system further comprises means for storing location information obtained by tracking said plurality of mobile stations and means for grouping any combination of said plurality of mobile stations which are caused to change the access system, in accordance with said location information.
In the radio communication system according to the present invention, said plurality of mobile stations have means for detecting interference caused by other adjacent mobile stations and notifying the base station of said interference, and wherein said base station is provided with means for deciding from said notification that the space-division access system becomes unavailable.
In the radio communication system according to the present invention, said change-over means including means for maintaining an information transmission rate per each of said mobile stations by increasing an information transmission rate per said radio communication system, when the space-division access system becomes unavailable.
In the radio communication system according to the present invention, said change-over means including means for notifying upper layers associated with an application and the user of the fact that a transmission rate within a radio zone is decreased, when the space-division access system becomes unavailable.
The present invention also provides a radio communication method used in a radio communication system which is provided with a plurality of mobile stations and a base station connected to the mobile stations via a radio, said plurality of mobile stations are served by the same base station and are using the same frequency, the method comprising the steps of detecting whether said plurality of mobile stations are closely located with one another and said base station is impossible to provide a space-division access system using an antenna-beam forming function and causing said plurality of mobile stations and said base station to change their access systems from the space-division access system to a time-division access system if it is detected in said detecting step that said base station is unable to provide the space-division access system, wherein said plurality of mobile stations and said base station successively use said same frequency for implementation of the time-division access system.
The radio communication method according to the present invention, further comprising the steps of storing location information obtained by tracking said plurality of mobile stations and grouping any combination of said plurality of mobile stations which are caused to change the access system, in accordance with said location information.
In the radio communication method according to the present invention, interference caused by other adjacent mobile stations is detected in said plurality of mobile stations and the base station is notified of said interference, and in said base station, the space-division access system is decided to become unavailable from said notification.
In the radio communication method according to the present invention, said step of causing to change the access system including a step of maintaining an information transmission rate per each of said mobile stations by increasing an information transmission rate per said radio communication system, when the space-division access system becomes unavailable.
In the radio communication method according to the present invention, said step of causing to change the access system including a step of notifying upper layers associated with an application and the user of the fact that a transmission rate within a radio zone is decreased, when the space-division access system becomes unavailable.